B. Sample PAM Module

The following is a minimal implementation of pam_unix(8), offering only authentication services. It should build and run with most PAM implementations, but takes advantage of OpenPAM extensions if available: note the use of pam_get_authtok(3), which enormously simplifies prompting the user for a password.

    
#include <sys/param.h>

#include <pwd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

#include <security/pam_modules.h>

#ifndef _OPENPAM
static char password_prompt[] = "Password:";
#endif

#ifndef PAM_EXTERN
#define PAM_EXTERN
#endif

PAM_EXTERN int
pam_sm_authenticate(pam_handle_t *pamh, int flags,
    int argc, const char *argv[])
{
#ifndef _OPENPAM
    struct pam_conv *conv;
    struct pam_message msg;
    const struct pam_message *msgp;
    struct pam_response *resp;
#endif
    struct passwd *pwd;
    const char *user;
    char *crypt_password, *password;
    int pam_err, retry;

    /* identify user */
    if ((pam_err = pam_get_user(pamh, &user, NULL)) != PAM_SUCCESS)
        return (pam_err);
    if ((pwd = getpwnam(user)) == NULL)
        return (PAM_USER_UNKNOWN);

    /* get password */
#ifndef _OPENPAM
    pam_err = pam_get_item(pamh, PAM_CONV, (const void **)&conv);
    if (pam_err != PAM_SUCCESS)
        return (PAM_SYSTEM_ERR);
    msg.msg_style = PAM_PROMPT_ECHO_OFF;
    msg.msg = password_prompt;
    msgp = &msg;
#endif
    for (retry = 0; retry < 3; ++retry) {
#ifdef _OPENPAM
        pam_err = pam_get_authtok(pamh, PAM_AUTHTOK,
            (const char **)&password, NULL);
#else
        resp = NULL;
        pam_err = (*conv->conv)(1, &msgp, &resp, conv->appdata_ptr);
        if (resp != NULL) {
            if (pam_err == PAM_SUCCESS)
                password = resp->resp;
            else
                free(resp->resp);
            free(resp);
        }
#endif
        if (pam_err == PAM_SUCCESS)
            break;
    }
    if (pam_err == PAM_CONV_ERR)
        return (pam_err);
    if (pam_err != PAM_SUCCESS)
        return (PAM_AUTH_ERR);

    /* compare passwords */
    if ((!pwd->pw_passwd[0] && (flags & PAM_DISALLOW_NULL_AUTHTOK)) ||
        (crypt_password = crypt(password, pwd->pw_passwd)) == NULL ||
        strcmp(crypt_password, pwd->pw_passwd) != 0)
        pam_err = PAM_AUTH_ERR;
    else
        pam_err = PAM_SUCCESS;
#ifndef _OPENPAM
    free(password);
#endif
    return (pam_err);
}

PAM_EXTERN int
pam_sm_setcred(pam_handle_t *pamh, int flags,
    int argc, const char *argv[])
{

    return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_acct_mgmt(pam_handle_t *pamh, int flags,
    int argc, const char *argv[])
{

    return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_open_session(pam_handle_t *pamh, int flags,
    int argc, const char *argv[])
{

    return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_close_session(pam_handle_t *pamh, int flags,
    int argc, const char *argv[])
{

    return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_chauthtok(pam_handle_t *pamh, int flags,
    int argc, const char *argv[])
{

    return (PAM_SERVICE_ERR);
}

#ifdef PAM_MODULE_ENTRY
PAM_MODULE_ENTRY("pam_unix");
#endif